The invention relates to an electroforming method for producing metal products having a pattern of openings separated by dykes using an electroforming mandrel in an electroplating bath, wherein metal from the electroplating bath is deposited on electrically conducting regions of the mandrel, which regions comprise at least two regions electrically insulated from one another.
Such a method is disclosed in NL-C.sup.6 -1001220. A method is described therein for growing electroplating products in which a mandrel is used which is composed of a plastic substrate, a so-called stencil pattern layer made of plastic and an electrical contacting layer. The stencil pattern layer of the mandrel has (at least partly) a surface roughness of less than 100 nanometers in order to facilitate the detachment of grown products. The contacting layer comprises mutually electrically insulated parts which are each provided with a separate connecting wire. Furthermore, the contacting layer is preferably applied in the base of the stencil pattern layer. With the aid of such a mandrel, an electroplated growth product can be made with at least two different thicknesses or from at least two different materials by connecting the connecting wires selectively to a current source at a suitable time or by using different electroplating baths.
A disadvantage of the above method is that, to obtain an electroformed product having different thicknesses, the switching times for the selective connection of the mutually electrically insulated parts to a current source have to be monitored accurately.
Mandrels known in the art are, for example, used to make, in particular, flat precision products such as, for example, ink jets, coding discs and coding strips, screens for microfiltration and the like. A high dimensional accuracy can be achieved in the case of these products because, during the electroforming, metal, for example, nickel, can be deposited with the same thickness everywhere or virtually everywhere over a relatively large surface area. Products having an accurate thickness can therefore be made by exactly controlling the factors comprising amperes per unit surface area and processing time.
Another known mandrel used in electroforming comprises an electrically conducting substrate to which a pattern of resist islands, separated by electrically conducting dykes, is applied with the aid of photosensitive resist. Said resist islands correspond with the openings to be formed in the product. During electroforming using such a mandrel, the latter is placed in an electroplating bath and connected as cathode, as a result of which metal from the bath is deposited on the electrically conducting regions. During the deposition, not only upward growth in the height direction (i.e. perpendicular to the mandrel), but also lateral overgrowth over the resist islands takes place. If the electroforming is continued for too long, the product becomes too thick and the openings may be overgrown, which is undesirable.
In many precision products, such as the examples given above, a high density (number/unit surface area) of small openings in the product is desired or required, as a result of which the products can be made only with a limited thickness. In same cases, depending on the density and the dimensions of the openings, only a thickness of a few .mu.m or even still less is possible. The disadvantage of such a small thickness is that, as a result, the products cannot be handled and, in addition, must not or cannot be mechanically loaded in an article or device assembled from such a product or during the assembly thereof.
In order to prevent the overgrowth of the openings and, nevertheless, to make the products with the required thickness, a multistage method is often used in practice in which a second much thicker growth is deposited on the first thin growth, the vulnerable regions, namely the openings being masked by photoresist. In fact, in this case, a second mandrel is placed on the first for the further growth. The final product acquires its strength, including rigidity, from the combined growth. However, this method is complex, expensive and complicated, inter alia because the second mandrel has to be positioned very accurately on the first one (alignment within a few .mu.m is required).